The fabrication of various assemblies, such as vehicle or machine frames, generally require a number of fabrication elements to be attached and welded to one another. One method of attaching fabrication elements in the proper orientation to one another during welding is known in the art as the slot and tab method. The slot and tab method, shown in co-owned U.S. Pat. No. 6,135,286, issued to Kelley et al., on Oct. 17, 2000, permits the attachment of fabrication elements to one another without the use of fixtures, or with the use of a reduced amount of fixtures, thereby reducing costs and increasing productivity.
The slot and tab method attaches a second fabrication element including a tab to a first fabrication element including a slot by inserting the tab into the slot so that a breakable portion of the tab protrudes through one side of the slot. The breakable portion of the tab can be rotated to secure the attachment. In order to create a fixed joint between the two fabrication elements, an interface between the two fabrication elements is fillet welded. The breakable portion is then detached from the tab by further rotating the breakable portion. A severed portion of the tab, including a tab shoulder, remains within the slot. In order to, at least partially fill a void within the slot, the slot is plug welded. Generally, the slot and the tab are rectangular and close fitting so that mating the tab and slot automatically aligns the fabrication elements for welding, often without the need for an external fixture.
Although the existing slot and tab design has performed well in many fabricated assemblies, the use of the slot and tab design has been limited to the fabricated assemblies having relatively low stress applications. Engineers have found that the current slot and tab design limits the penetration depth of the fillet weld and the plug weld, thereby creating unfused areas within the fabrication assembly. These unfused areas can create small voids that remain imbedded within the attachment after the fabrication assembly is completed. When the fabrication assembly is subjected to repeated loads, stress can concentrate around the voids, sometimes causing the small voids to become initiation points for fatigue cracks. Further, engineers have found that, due to the abrupt change in stiffness from the fillet weld to the plug weld, the plug weld within the slot can act also act as a stress concentrator. Rather than being distributed through the fabrication assembly, stress can concentrate within the plug weld, especially around the sharp corners of a rectangular plug weld. Again, the stress can lead to fatigue cracks. The fatigue cracks can lead to premature failure of the fabrication assembly.
Referring to FIGS. 1–3, there are shown sectioned side views of a welded joint 9 with a plug weld 14 removed, according to the prior art. In FIGS. 1–3, a second fabrication element 12 including a tab shoulder 11 and a severed neck 20 is oriented perpendicularly to a first fabrication element 10 defining a slot 13. The tab shoulder 11 and the severed neck 20 are positioned within the slot 13. A fillet weld 15 runs along an interface 16 between the first fabrication element 10 and the second fabrication element 12. The plug weld 14 is removed from the slot 13 for illustration purposes, although it should be appreciated that the plug weld 14 is positioned within the slot 13 to form the welded joint 9. The slot 13 and the tab shoulder 11 are rectangular, and the tab shoulder 11 is approximately half the height of the walls of the slot 13.
Referring specifically to FIG. 1, there is shown a first crack plane 17 between the first and the second fabrication elements 10 and 12. The first crack plane 17 is parallel to the fillet weld 15. The first crack plane 17 is caused by an unfused area between the first fabrication element 10 and the second fabrication element 12. The unfused area is caused by a limited penetration of the fillet weld 15. When load is applied to the welded joint 9, the portion of the welded joint 9 in the first crack plane 17 will be subjected to the stress of the load that cannot be distributed through the unfused area, thereby creating small voids within the first crack plane 17. After repeated loads, the small void can eventually propagate outward as fatigue cracks, leading to premature failure of the welded joint 9.
Referring now to FIG. 2, there is shown a second crack plane 18 between the relatively long sides of the rectangular tab shoulder 11 and the slot 13. The second crack plane 18 runs along the relatively long sides of the tab shoulder 11 parallel to the fillet weld 15, and is caused, in part, by the limited penetration of the fillet weld 15. If the fillet weld 15 does not penetrate up to the plug weld 14, there will be an unfused surface between the fillet weld 15 and the plug weld 14 Further, there may be an unfused surface extending into the slot 13. Because the tab shoulder 11 generally extends, at least half way through the slot 13 and is rectangular with relatively sharp corners, the penetration of the weld material between the relatively long sides of the shoulder 11 and the slot is limited, thereby leaving another unfused surface or void. The unfused surfaces have been found to act as a stress concentrator in the second crack plane 18 from which fatigue cracks may form.
Referring to FIG. 3, there is shown a third crack plane 19 between the slot 13 and the relatively short sides of the tab shoulder 11 that run perpendicular to the fillet weld 15. The third crack plane 19 is caused by unfused surfaces between the relatively short sides of the tab shoulder 11 and the slot 13. Again, because of the shape and size of the tab shoulder 11 and the slot 13, there can be limited penetration of the plug weld material between the sides of the tab shoulder 11 and the slot 13. Further, the plug weld 14 may not make contact with a top surface of the tab shoulder 11 from which the severed neck 20 protrudes, thereby leaving a void between the plug weld 14 and the tab shoulder 11. Again, these unfused surfaces, caused by the limited penetration of the plug weld 14, can lead to the formation of the third crack plane 18. The thin voids created by the unfused surfaces can act as initiation point for fatigue cracks.
The present invention is directed at overcoming one or more of the problems set forth above.